The present invention relates to improved methods for making fluid diffusion layers and electrodes having reduced surface roughness and methods for making membrane electrode assemblies having better reliability and performance. The methods comprise adhering at least one loading material to a porous substrate in a manner such that the surface roughness of the resulting fluid diffusion layer is reduced. The reduced surface roughness may be assessed in terms of average surface roughness (Ra) or by observation of infrared hot-spots detected.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrolyte fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrically conductive electrodes. Such electrodes comprise a fluid diffusion layer and an electrocatalyst. The fluid diffusion layer comprises a substrate with a porous structure having voids therein. The substrate is permeable to fluid reactants and products in the fuel cell.
The electrocatalyst is typically disposed in a layer at each membrane/electrode interface, to induce the desired electrochemical reaction in the fuel cell. The electrocatalyst may be disposed as a layer on the electrode or be part of the electrode in some other way. The electrocatalyst may be disposed on the membrane instead of or in addition to being disposed on the fluid diffusion layer. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
Fluid reactants may be supplied to the electrodes in either gaseous or liquid form. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) and electrons from the fuel. The gaseous reactants move across and through the fluid diffusion layer to react at the electrocatalyst. The ion exchange membrane facilitates the migration of protons from the anode to the cathode while electrons travel from the anode to the cathode via the external load. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane and the electrons to form water as the reaction product.
In solid polymer electrolyte fuel cells employing methanol as the fuel supplied to the anode (so-called xe2x80x9cdirect methanolxe2x80x9d fuel cells) and an oxygen-containing oxidant stream such as air (or substantially pure oxygen) as the oxidant supplied to the cathode, methanol and water are oxidized at the anode to produce protons and carbon dioxide. Typically, the methanol is supplied to the anode as an aqueous solution or as a vapor. Gaseous or liquid reactants move across and through the fluid diffusion layer. The protons migrate through the ion exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, oxygen reacts with the protons and electrons to form water.
In solid polymer electrolyte fuel cells, the MEA is typically interposed between two separator plates or fluid flow field plates (anode and cathode plates). The plates typically act as current collectors and provide support to the MEA. Fluid flow field plates typically have channels, grooves or passageways formed therein to provide means for access of the fuel and oxidant streams to the porous fluid diffusion layers of the anode and cathode, respectively.
The electrode is electrically conductive to provide a conductive path between the electrocatalyst reactive sites and the current collectors. Materials commonly used as substrates of electrodes or as starting materials to form substrates include carbon fiber paper, woven carbon fabric, optionally filled with carbon particles and a binder, metal mesh or gauze, optionally filled with carbon particles and a binder, and other woven and nonwoven materials.
Typical substrate materials are preformed, highly electrically conductive macroporous sheet materials, which may contain a particulate electrically conductive material and a binder. It has sometimes been found advantageous to coat, impregnate, fill, or otherwise apply porous electrically conductive substrates with materials, such as carbon or graphite materials, in order to reduce porosity or achieve some other object. The material applied to the substrate is referred to herein as xe2x80x9cloading material.xe2x80x9d When loading material is applied to one side of a substrate to form a layer, the formed layer is frequently referred to as a xe2x80x9csublayerxe2x80x9d. The amount of loading material (that is, the material eventually loaded onto the substrate) in an electrode is referred to as the xe2x80x9cloadingxe2x80x9d of loading material and is usually expressed as the mass of material per unit surface area of substrate.
A certain loading of carbon or graphite can improve MEA operational performance. However, if the loading is too high, performance is impaired by interference with diffusion of product or reactant through the fluid diffusion layer. Nonetheless, substrates having larger pores or a higher porosity tend to require higher loadings of carbon or graphite.
A substrate need not be highly electrically conductive and in fact may be an electrical insulator. Such substrates may be filled with electrically conductive materials. Electrodes that are made from filled, poorly electrically conductive webs and methods for making same are disclosed in U.S. Pat. Nos. 5,863,673 and 6,060,190, which are incorporated herein by reference.
A substrate for an electrode typically has a loading material applied to it in order to provide a supporting surface for electrocatalyst, to improve conductivity, and/or to accomplish some other objective. The loading material can be applied by any of the numerous coating, impregnating, filling or other techniques known in the art. The loading material may be contained in an ink or paste that is applied to the substrate. In a typical process for applying a loading material to substrate, the substrate has an ink applied to it, and the ink comprises carbon and/or graphite with a poreformer and a binder (for example, polytetrafluoroethylene) in aqueous solution. After this application, the substrate and the loading material applied to the substrate may or may not be subjected to compaction at an elevated pressure, such as the pressure to which the electrode may be subjected in a fuel cell stack or a higher pressure. The substrate and applied loading material are dried, with the result that the substrate is loaded to a greater or lesser extent with the loading material on its surface and/or within the voids, thus forming a fluid diffusion layer. Binder in the fluid diffusion layer is typically sintered before the electrocatalyst is applied. The final fluid diffusion layer is still permeable to fluid reactants.
U.S. Pat. No. 6,127,059 discloses a gas diffusion layer for use in a solid polymer electrolyte fuel cell that makes use of a membrane electrode assembly of the type in which a catalyst layer is formed on the surface of a solid polymer electrolyte membrane. The gas diffusion layer includes a carbon fiber woven cloth having a surface and a coating of fluororesin containing carbon black on the surface. Preferably, the coating penetrates no more than one-half, more preferably no more than one-third, the thickness of the carbon fiber woven cloth. The carbon fiber woven cloth may be pre-treated with a water-repellent fluororesin (such as polytetrafluoro-ethylene), or with a mixture of a fluororesin and carbon black, to enhance water repellency. U.S. Pat. No. 6,127,059 does not disclose or suggest the step of compacting the carbon fiber woven cloth after applying the coating of fluororesin.
Compaction has been used in other processes of loading a material upon a substrate. Compaction of the wet coated porous substrate tends to push the loading material into the substrate.
Fluid diffusion layers have been made using release materials, such as Mylar release films. In some cases, the release film has a loading material applied to one surface, and then a substrate is applied over the release film. Mylar sheets have been used as release films. This combination of substrate, loading material, and release film is dried and heated, after which the release sheet is peeled off. U.S. Pat. No. 6,127,059 discusses the use of a release sheet.
In the fabrication of membrane electrode assemblies for solid polymer fuel cells, the detection of perforations or leaks in the membrane is an important aspect of quality control because of the need during fuel cell operation to maintain fluid isolation of the fuel and oxidant streams and electrical isolation of the electrodes. A perforation in the membrane may result in fluid transfer leaks across the membrane and/or electrical contact between the electrodes, causing a short-circuit. Fluid transfer may arise even where there is no perforation, such as when the membrane thins so much that it does not adequately prevent reactants from permeating through the membrane. A leak in the membrane of a fuel cell can cause the fuel and oxidant streams to fluidly communicate and chemically react, thereby degrading the electrochemical potential of the fuel cell. Fluid communication of the fuel and oxidant streams through a leak in the membrane during fuel cell operation can also result in serious degradation of the membrane due to the combustion of the fuel in the presence of catalyst and oxygen.
Leaks in a membrane may be detected by detecting the heat generated by an exothermic reaction of a pair of reactants, which are normally substantially isolated on opposite sides of the membrane, and which contact each other and react only if there is a transfer leak present. With the use of an appropriate heat detector, such as a thermal imaging device, the location of the perforations and leaks in the membranes may be determined, as they are located at the location of the exothermic reaction. The locations of exothermic reaction as seen by a thermal imaging device are referred to as xe2x80x9chot-spots.xe2x80x9d
The ability to locate perforations or leaks in the membrane after fabrication of the MEA can assist in diagnosis of the cause of the perforations. Techniques for detecting perforations in membranes by detecting exothermically generated heat are disclosed in U.S. Pat. No. 5,763,765, which is incorporated herein by reference. Further, the ability to detect electrodes likely to cause perforations or leaks in the membrane of an MEA before the MEA is assembled can provide a manufacturing advantage.
General techniques useful for evaluating the roughness of a surface include qualitatively or quantitatively measuring the surface such as by optical surface analysis. A Wyko optical surface analyzer is a suitable instrument for optical surface analysis and provides a profile of the surface along a line or plane. Other techniques for surface profiling include a Surftest stylus, which has a tip that is dragged over the measured surface at light pressure (like a record needle) to give a two-dimensional surface profile.
One measure of surface roughness is the average surface roughness (xe2x80x9cRaxe2x80x9d). Ra is a standard surface profile parameter used in the surface finishing industry (see for example, the Surface Metrology Guide developed by Precision Instruments Inc., ASME B46.1-1995, ASME B46.1-1985, ISO 4287-1997, and ISO 4287/1-1984) and is defined as the area between the roughness profile of a surface and its mean line, or the integral of the absolute value of the roughness profile height over the evaluation length. Note that Ra measures the profile of a section of a surface (i.e. is a two-dimensional measurement); in contrast, the xe2x80x9caverage Raxe2x80x9d relates to the three-dimensional topography of a surface and is the averaged value of a plurality of sectioned profiles of the surface. While Ra values are one way to quantify surface roughness, other measurable industry standard surface profile parameters such as peak count (Pc) or kurtosis (Rku) may be used, and have been found to correlate with experimental results.
Improved methods of preparing a fluid diffusion layer having reduced surface roughness are provided. An improved fluid diffusion layer is also provided. An improved method of making a membrane electrode assembly having better reliability and performance is also provided. An improved method of assessing the surface roughness of a fluid diffusion layer or an electrode is further provided.
It has been found desirable to reduce the surface roughness of a fluid diffusion layer, particularly as reflected by its average Ra. Several improved methods are disclosed for preparing a fluid diffusion layer comprising a substrate and a loading material adhered to the substrate. Each of these methods independently or in combinations thereof can improve surface roughness.
In one method, a loading material is adhered to the substrate by the steps of (a) applying a first loading composition comprising a first portion of the loading material to the substrate in a first applying step; and (b) applying a second loading composition comprising a second portion of the loading material to the substrate in a second applying step. The substrate and the loading material applied thereto may be compacted after one or both applying steps.
In another method, a loading material is adhered to a substrate by the steps of (a) applying a loading composition comprising a loading material to the substrate, (b) partially drying the substrate and the loading composition applied thereto in a first drying step, (c) compacting the substrate and the loading material applied thereto in a compacting step, and (d) further drying the substrate and the loading material applied thereto in a second drying step.
In yet another method, a loading material may be adhered to a substrate by the steps of (a) applying a loading composition to at least one of a substrate and a release material so that at least one coated surface is formed; (b) contacting the substrate and the release material such that the coated surface is disposed between the substrate and the release material; (c) compacting the substrate, the release material, and the loading composition in a compacting step, (d) drying the substrate, the release material, and the loading composition in a drying step, and (e) removing the release material from the substrate and the loading material.
Through the use of the foregoing methods, fluid diffusion layers having reduced surface roughness may be prepared. The fluid diffusion layers and electrodes described herein are surprising in having reduced surface roughness even with a lower average amount of loading material than conventional fluid diffusion layers.
As another aspect, a method for preparing a membrane electrode assembly for an electrochemical fuel cell is also provided. The membrane electrode assembly comprises a pair of fluid diffusion layers and an ion exchange membrane interposed between the fluid diffusion layers. Electrocatalyst is disposed at the interface of each fluid diffusion layer with the ion exchange membrane. The method comprises the steps of forming one or both fluid diffusions layers according to any of the foregoing methods, and consolidating the pair of fluid diffusion layers and the ion exchange membrane into a unitary membrane electrode assembly having two major planar surfaces.
As yet another aspect, a method for evaluating surface roughness of a fluid diffusion layer or an electrode provided. The method comprises detecting infrared hot-spots corresponding to the exothermically generated heat in a fashion similar to that described in U.S. Pat. No. 5,763,765, and calculating the average Ra value of the fluid diffusion layer based upon the detecting of the infrared hot-spots.
A novel fluid diffusion layer comprises a porous substrate and a loading material adhered to the substrate, wherein the fluid diffusion layer comprises loading material in an average amount of about 3 mg/cm2 or less, preferably about 2.3 mg/cm2 or less, and by reduced surface roughness reflected by an average Ra of less than about 13 xcexcm, preferably less than about 10 xcexcm. As used herein, the xe2x80x9caverage amountxe2x80x9d refers to the mass of loading material per unit surface area of substrate. The substrate is preferably characterized prior to the application of the loading material as having a porosity of at least about 80%, an average pore size of about 30 xcexcm or greater, and/or an average Ra of about 16 xcexcm or greater.